There have been known various types of oxygen sensors such as a galvanic cell type sensor (fuel cell type sensor), a polarograph type sensor, a paramagnetic wind type sensor, and a solid zirconia electrolyte type sensor. Of these sensors, the galvanic cell type oxygen sensor is generally simple and cheap and can work at room temperature, so that it is used in wide areas of application.
The galvanic cell type oxygen sensor comprises a galvanic cell comprised of a cathode made up of metal effective for the electrolytic reduction of oxygen such as platinum, gold or silver, an anode made up of lead and an electrolyte, and the sensor makes use of the linear relation between an oxygen concentration and an electric current flowing between the cathode and the anode when a certain resistor is connected between the cathode and the anode.
In the galvanic cell type oxygen sensor, the cathode, the anode and the electrolyte are housed in a cell container. A part of the cell container is made up of a polymer film. This polymer film is partially in intimate contact with the cathode and functions to control appropriately the diffusion rate of oxygen which permeates through the polymer film and reaches the cathode surface.
It is usual that a thermistor for temperature compensation is further connected between the cathode and the anode in addition to the resistor.
The conventional galvanic cell type oxygen sensor had such a fatal defect that its life was as short as 10 to 12 months. The short life of the conventional sensor was caused by use of, as the electrolyte, an aqueous solution of potassium hydroxide or sodium hydroxide.
With a galvanic cell type oxygen sensor employing an alkaline electrolyte, the electrolytic reduction of oxygen as shown in equation (1) occurs at the cathode, whereas the reaction as shown in equation (2) occurs at the anode. EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- ( 1) EQU 2Pb+4OH.sup.- .fwdarw.2PbO+2H.sub.2 O+4e.sup.- ( 2)
PbO that is a reaction product of the anode becomes dissolved into the electrolyte and thus, the surface of the lead electrode is always renewed. In such a state, since the potential of the anode is stabilized, the galvanic cell type oxygen sensor works normally. But when the electrolyte is saturated with the reaction product of the anode, the anode surface is passivated, and the overvoltage of the anode is increased. Thus, the electric current flowing between the cathode and the anode changes, and the linear relation between the oxygen concentration and the electric current breaks down, which results in ending the life of the oxygen sensor.
The reason why the life of the conventional galvanic cell type oxygen sensor employing an alkaline electrolyte was so short resides in that the solubility of PbO as the reaction product in the alkaline electrolyte is so small as about 0.1 mol/l at maximum.
On the other hand, it has been also known that the life of the galvanic cell type oxygen sensor employing an alkaline electrolyte is further shortened when the sensor is placed in an atmosphere containing a relatively high concentration of carbon dioxide. That is because carbon dioxide permeates through the polymer film to be dissolved in the electrolyte and forms insoluble lead carbonate (PbCO.sub.3) or basic lead carbonate [Pb.sub.2 CO.sub.3 (OH).sub.2 ] instead of PbO formed according to the above equation (2) at the anode, which results in markedly increasing the overvoltage of the anode.
Japanese Patent Application (OPI) No. 53891/1974 discloses that acetic acid can be used as an electrolyte for the galvanic cell type oxygen sensor. The term "OPI" as used herein refers to a "published unexamined Japanese patent application".
When an aqueous solution of acetic acid is used as the electrolyte, the reaction as shown in equation (3) occurs at the cathode, whereas the reaction as shown in equation (4) occurs at the anode. EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O (3) EQU 2Pb+2H.sub.2 O.fwdarw.2PbO+4H.sup.+ +4e.sup.- ( 4)
The reaction product of the anode is also PbO as in the case of using the alkaline electrolyte. The solubility of PbO in the aqueous solution of acetic acid is 2.1 mol/l, which is about 20 times larger than that of PbO in the alkaline electrolyte. Therefore, it could be inferred that the oxygen sensor employing an acetic acid solution as the electrolyte has an extremely long life. However, the oxygen sensor employing an acetic acid solution as the electrolyte has not ever been put into practice uses, and there have not been found any literatures about the life of the sensor. This is because the conductivity of the aqueous solution of acetic acid is as small as 16.times.10.sup.-4 .OMEGA..sup.-1.cm.sup.-1 at a concentration of 3 mol/l at 18.degree. C., and the internal resistance of the oxygen sensor becomes too high.
Another reason why the acetic acid solution has not been put into practice uses as the electrolyte resides in that there is a possibility that hydrogen generates from the cathode. When the oxygen sensor is placed in an atmosphere having an oxygen concentration near zero, the cathode and the anode have inevitably almost the same potential because they are connected through the resistor. Therefore, unless the potential of the lead anode, i.e., the potential of the cathode, is made nobler than an equilibrium potential of the cathode for hydrogen generation, hydrogen likely generates from the cathode. The equilibrium potential of the cathode for hydrogen generation is provided by equation (5). ##EQU1## wherein E.sub.H is an equilibrium potential for hydrogen generation at 25.degree. C.; P.sub.H.sbsb.2 is a partial pressure of hydrogen; and pH is a hydrogen ion concentration in the electrolyte.
In the equation (5), when hydrogen generates in the form of bubbles from the cathode, P.sub.H.sbsb.2 equals 1, so that the equation (5) is transformed into equation (6). EQU E.sub.H =-0.2412-0.05916pH (6)
In the equation (6), the lower the pH, the nobler will become the equilibrium potential of the cathode for hydrogen generation and thus the larger will become the possibility of hydrogen generation from the cathode.
When a solution having a low pH such as an acetic acid solution is used as the electrolyte, the equilibrium potential of the cathode for hydrogen generation becomes very noble, and the hydrogen generation from the cathode occurs almost certainly.